Nowadays, there is a great demand for weight reduction in automobiles from the viewpoint of fuel economy, environmental protection, etc. As a means for achieving this, use of propeller shafts formed of FRP (fiber-reinforced plastics) are being considered, and some of them have already been put into practical use. Such an FRP propeller shaft has a cylindrical main body that is made of FRP, and metal joints that are joined to the ends of this main body.
An automobile propeller shaft, which serves to transmit torque generated in the engine to driving wheels, is required to have a torsional strength of approximately 100.about.400 kgf.m. Further, it is also required to have a critical revolution of approximately 5,000 to 15,000 rpm in order that resonance may be avoided in high-speed rotation. To satisfy these fundamental requirements, various parameters, such as the kind, quantity and orientation of reinforcing fibers, the layered structure, the outer and inner diameters, and the wall thickness, are taken into consideration when designing the main body, which is made of FRP.
For example, in determining the orientation of the reinforcing fibers, the following facts are to be taken into account: mainly from the viewpoint of torsional strength, the reinforcing fibers are most effectively arranged at an angle of .+-.45.degree. with respect to the axial dimension of the main body. Mainly from the viewpoint of torsional buckling strength, the most effective angle of arrangement for the reinforcing fibers is .+-.80.about.90.degree. with respect to the axial dimension of the main body. Mainly from the viewpoint of critical revolution, the reinforcing fibers are to be arranged in a direction as close as possible to the axial direction in order to achieve an increase in bending elasticity modulus to thereby obtain a high bending resonance frequency.
Thus, the most effective orientation for the reinforcing depends upon the fundamental requirement to be taken into consideration, such as torsional strength or critical revolution, which means the layer structure has to be determined by appropriately combining orientations that are most suitable from the viewpoint of the actual requirements. The torsional strength can also be dealt with in terms of dimensions, such as outer diameter and wall thickness, so that, when designing a propeller shaft, first priority is usually given to the critical revolution, which greatly depends upon the orientation of the reinforcing fibers, and the proportion of those layers in which the reinforcing fibers are arranged at a small angle with respect to the axis of the shaft is made relatively large. This, however, entails the following problems:
The assurance of safety for the passengers when a collision occurs is an issue no less important than weight reduction. The prevailing present-day idea in automobile design regarding safety assurance consists in a crashable body structure, in which the impact energy (compressive load) at the time of collision is absorbed by the compressive destruction of the body, thereby mitigating the rapid acceleration applied to the passengers. It should be noted, however, that, if the body of the FRP propeller shaft is designed in conformity with the above idea, which gives priority to critical revolution, the strength of the body with respect to an axial compressive load must inevitably increase. This leads to a deterioration in the impact energy absorbing effect. Thus, when the body suffers rupture as a result of a collision and the rupture proceeds to reach the propeller shaft, the propeller shaft will act as a kind of prop.
As a means for solving this problem, Japanese Patent Laid-Open No. 3-37416 proposes a propeller shaft in which the joints are allowed to move axially along the joint surfaces between the main body and these joints, and, in this process, the joints force the main body to gradually enlarge until its rupture, starting from the ends thereof, thereby breaking the propeller shaft. However, in this conventional propeller shaft, it is necessary for the main body and the joints to be joined together through the intermediation of teeth of a complicated shape, a separating agent, etc., in order to secure the movement of the joints, resulting in a rather complicated structure. Furthermore, a complicated production process is not avoided. Moreover, when, in a propeller shaft having such a construction, joints are to be joined by press fitting, the main body must be strong enough to withstand the force applied in the press fitting process. However, imparting such a high strength to the main body makes it difficult for the main body to be enlarged and broken by the compressive load. Thus, it is quite difficult simultaneously to satisfy the above-mentioned fundamental requirements and the requirements regarding enlargement and rupture, which are contradictory to each other.
Japanese Patent Laid-Open No. 4-339022 discloses a propeller shaft in which, when an axial compressive load is applied, the joints are caused to move along the joint surfaces between the main body and these joints toward the interior of the main body, whereby the impact energy is absorbed by the movement resistance. However, in such a construction, it is absolutely necessary for the outer diameter of the joints to be smaller than the inner diameter of the main body, resulting in a reduction in the degree of freedom in designing. Furthermore, the amount of movement is limited to the length of the joints, so that the effect of absorbing the impact energy is not so great.
Thus, the conventional propeller shafts can not be regarded as well balanced in terms of fundamental requirements regarding torsional strength, critical revolution, etc. and safety assurance for the passengers at the time of a collision.